The invention relates to a method for fluorination of a soot body, fluorinated soot bodies obtained by the method, a method for producing a synthetic glass for an optical element using this fluorination method, as well as glass obtained by this production method.
When producing synthetic quartz glass for commercial applications, it is customary to generate SiO2 particles from a silicon-containing starting substance in a CVD (chemical vapor deposition) procedure by hydrolysis and/or oxidation, followed by precipitation on a support. The method can be subdivided into external and internal deposition methods. In external deposition methods, SiO2 particles are applied onto the outside of a rotating support. Examples of depositing methods include the so-called OVD method (outside vapor phase deposition) and the PECVD method (plasma-enhanced chemical vapor deposition). The most prominent example of an internal deposition method is the MCVD method (modified chemical vapor deposition), in which SiO2 particles are deposited on the internal wall of a tube that is heated from outside. If the temperature at the support surface is sufficiently high, the SiO2 particles become vitrified directly (“direct vitrification”).
In contrast, in the so-called “soot process,” the temperature gets so low during the deposition of the SiO2 particles that a porous soot layer is obtained, which is then sintered into a transparent glass in a separate process step. One pertinent example is the “OVD process” known from DE 10 2007 024 725 A1, in which combustion gases in the form of hydrogen and oxygen and a silicon-containing starting compound are fed to a depositing burner and are then converted into SiO2 particles in a burner flame allocated to the depositing burner. These particles are then deposited layer-by-layer by back-and-forth motions of the depositing burner along a substrate that rotates along its longitudinal axis, resulting in the formation of an SiO2 blank.
Synthetic quartz glass for use in microlithography or for optical components in telecommunications must meet strict requirements in terms of the homogeneity of its refractive index.
It is known from the prior art to use fluorine as a doping agent to lower the refractive index of quartz glass. Accordingly, US 2001/0018835 describes the production of UV-resistant, F-doped quartz glass, in which soot bodies are heated in an atmosphere consisting of hydrogen or oxygen and a sintering process takes place in a fluorine-containing atmosphere in a subsequent process step. This two-step treatment is to attain an improvement of the UV transmission.
JP 63-225543 A describes fluorine doping and vitrification of a porous silicon oxide-containing preform aiming to prevent structural defects.
EP 1 337 483 A1 describes a method for dehydrogenating a soot mold, in which a gas mixture of chlorine and carbon monoxide is applied to a soot body.
JP 62-176937 describes a method for producing fluorine-doped quartz glass in which a soot body is first treated with silane (SiH4) in an oxygen-depleted atmosphere in a first step in order to generate oxygen defects, which are to lead to an improved formation of SiF4 in a downstream fluorination step. This is to attain a higher degree of fluorination in the silicon oxide soot body.
In order to render the doping process as extensive and as uniform as possible, a doping gas that allows for reaction of the fluorine contained therein with the SiO2 matrix in an acceptable temperature window and diffuses into the soot body as quickly as possible needs to be used for the SiO2 soot bodies. However, it is a drawback that the soot body begins to sinter with increasing degree of doping during the doping process at the high temperatures used for doping. As a result, the pores close and the desired diffusion of the fluorination agent is made more difficult, which results in inhomogeneities due to the pre-sintering and in partially deformed soot bodies.
It has been found that tetrafluoromethane and hexafluoroethane can be used well for fluorination of SiO2-containing soot bodies, since these fluorohydrocarbons, unlike the common fluorination agent SiF4, show good handling properties and no toxicity, are inert at room temperature and are comparatively inexpensive. However, it is disadvantageous that the use of tetrafluoromethane or hexafluoroethane requires relatively high temperatures for thermal decomposition, and is associated with the formation of a reactive fluorine-containing entity. At these high temperatures, there is already substantial sintering of the soot body ongoing, which counteracts homogeneous fluorination. Moreover, it has been found that the use of the fluorohydrocarbons at the temperatures needed to attain high fluorine contents in the glass is associated with the generation of carbon, which can be deposited in the doping apparatus, but also in the soot body and might lead to blackening of the glass in an extreme scenario. The generation of soot during the fluorination with tetrafluoromethane or hexafluoroethane has a disadvantageous effect on the method and the quality of the soot bodies and/or quartz glass thus made.